1. Field of the Invention
The present invention relates to an electric contact and a method for producing the same. More particularly, the present invention relates to an electric contact of, for example, an electric wiring terminal, a relay or a switch used in an automobile, an industrial apparatus and the like, in particular, an electric contact which is used in a field where a long-term deterioration will be a problem or a use temperature is expected to rise, and a method for producing such electric contact.
2. Description of the Related Art
In an electric circuit, parts of a pair of conductors to be electrically connected, namely electric contacts should satisfy various property requirements. Among them, important ones are a low contact resistance, a high melting point, good resistance to fusing, and the like. When the electric contact is used for a long time and exposed to wind and rain or a high temperature or when it is required to have high reliability, it should be highly resistant to corrosion, abrasion and heat.
Hitherto, a terminal fitment to be used in an automobile or an industrial apparatus utilizes an electric contact made of a copper alloy such as brass, phosphor bronze or, in some cases, stainless steel or an electric contact made of copper or a copper alloy a surface of which is plated with tin or nickel. In a computer control circuit in which breakage should be avoided and through which a very weak current passes, an electric contact made of stainless steel plated with gold is used.
The copper alloy electric contact is cheap but poor in resistance to oxidation, so that, after long-term use, it is oxidized and its contact resistance increases.
The copper or copper alloy electric contact plated with tin or nickel has better oxidation resistance than the copper alloy electric contact but a relatively high contact resistance. In addition, it has poor resistance to chemicals.
The gold-plated electric contact is excellent in oxidation resistance and corrosion resistance and its contact resistance is stable at room temperature. Therefore, it is widely used in the fields where high reliability is required. However, when a pure gold plating is heated to a temperature of 90.degree. C. or higher, its adhesion and cohesion increase, so that it tends to be easily abrased by an opposite conductor to which it is contacted. In case of a gold plating which is hardened with an impurity, the impurity separates out onto the surface from grain boundaries and the contact resistance increases. Then, the gold plated electric contact has poor heat resistance. Further, gold is expensive.
Although stainless steel which is often used as a base material for gold plated electric contact has no problem in heat resistance, its resistivity is large, for example, about 70 .mu..OMEGA..cm in case of SUS 304. Further, since a surface of stainless steel has a passive state chromium oxide layer, the contact resistance is large.
When a pair of conductors to be electrically connected are made of different metals, electric erosion occurs between them in each case where the conductors are those of the electric contact which are contacted and separated or where they are connecting terminals which are fixed together.
The electric erosion occurs when water or other liquid penetrates between a pair of different metals to be contacted and a more base metal is dissolved from its contacted surface in water or the liquid due to a difference of corrosion potentials between the metals.
A combination of different metals is seen when an electric part or a body of an automobile or an air craft is made of a light aluminum material in view of a recent requirement for light weight while a cheap copper base material is used as an electrically conductive material, for example, when a grounding terminal made of a copper base material is connected to an aluminum body of an automobile. Since aluminum is a base metal, it almost always suffers from electric erosion if the terminal to be connected thereto is not made of aluminum.
Hitherto, to prevent the electric erosion at the contacting part between the different metals, the corrosion potential between the contacted metals is lowered to suppress the electric erosion by using a tin-plated copper alloy as a material of the copper base material so as to interpose a tin layer between the different metals. Alternatively, the contacting part of the different metal is sealed with a resin to prevent penetration of water or other liquid which causes the electric erosion into the contacting part.
When the metal material plated with, for example, tin is used, an amount of electric erosion can be reduced but the occurrence of electric erosion cannot be perfectly prevented. When the contacting part of the different metals is used in an corrosive atmosphere where deposition of moisture such as condensation caused by change of environmental conditions and used in a part which requires long term durability, for example, an automobile electrical part, an amount of electric erosion is not negligible. Further, the electrical material made of copper or a copper alloy, a surface of which is plated with tin or nickel, has better oxidation resistance than the copper alloy as such but it has relatively high contact resistance and poor resistance to chemicals.
When the contacting part of the different metals is sealed with the resin to prevent penetration of water, a presently used water-resistant resin has poor wettability with the metal so that the penetration of water at an interface between the metal and the resin cannot be completely prevented, and therefore the occurrence of electric erosion cannot be prevented. When a pair of the conductors are usually separated and contacted at the time of functioning, the conductors cannot be sealed with the resin.